1. Field of the Invention
The present disclosure generally relates to an image forming apparatus having a development unit, which develops a latent image on a latent image carrier using toner, and a process cartridge having the development unit.
2. Description of the Background Art
An image forming apparatus such as a copier, a plotter, a facsimile, or a multifunctional apparatus uses electrophotography for an image forming operation, for example. Such an image forming apparatus includes a development unit, which employs a two-component developing method using two-component developing agent including toner and magnetic carrier particles, or one-component developing method, which employs only toner as developing agent.
The two-component developing method, suitable for higher speed developing, is widely used for image forming apparatuses used for middle to high speed printing. In such two-component developing method, the developing agent needs to densely exist on a developing roller to produce a higher quality image, in which toner is transferred to a latent image on a latent image carrier. In light of such demand for higher quality image, smaller-sized carrier particles have been employed, and carrier particles having a diameter of about 30 μm are now available, for example.
The one-component developing method, suitable for reducing the size of developing mechanism, is widely used for image forming apparatuses used for low speed printing. In such a one-component developing method, a toner regulating member, such as for example a blade or a roller, is used to form a thin layer of toner on a developing roller. The toner is charged by the developing roller or the toner regulating member with an effect of frictional pressure. The thin layer of charged toner on the developing roller is transported to a developing area to develop a latent image on the latent image carrier. Such developing method may be a contact type and a non-contact type. In the contact type, a developing roller and a latent image carrier contact each other. While in the non-contact type, a developing roller and a latent image carrier do not contact each other.
As for the two-component developing method, a demand for higher quality image is growing in which an image dot size may need to be same or smaller than carrier particle diameter. Accordingly, a size of carrier particle needs to be further reduced to reproduce each one of image dots precisely. However, smaller-sized carrier particles cause several drawbacks. For example, carrier particles having smaller diameter have lower magnetic permeability, which may result into separation or drop-off of carrier particles from a developing roller. If such separated carrier particles adhere on a latent image carrier, a defective image may be produced, and the carrier particles may further physically damage the latent image carrier, for example.
Such carrier separation may be prevented by increasing the magnetic permeability of carrier particle or by increasing the magnetic force of a magnet installed in a developing roller. However, such attempts may have difficulties in achieving lower manufacturing cost and higher quality image at the same time. Further, with a growing demand for reducing a size of an image forming apparatus, a size of developing roller has been reduced. Thereby a developing roller having a strong magnetic field for preventing the carrier separation is difficult to devise.
In the two-component developing method, magnetic brushes composed of two-component developing agent are formed on a developing roller and the magnetic brushes are contacted against a latent image to form a toner image, wherein distribution of the magnetic brushes may have some unevenness across the developing roller, and thereby image dots may not be reproduced uniformly. Although an image quality can be enhanced by forming an alternating electric field between the developing roller and the latent image carrier, uneven distribution of magnetic brushes itself may not be eliminated completely.
In the one-component developing method, a thin layer of toner is formed on a developing roller by using a toner regulating member, by which such toner is strongly pressed against the developing roller, and thereby such toner may not respond to an electric field of a developing area so quickly. Accordingly, to obtain a higher quality image, an alternating electric field may be generated between the developing roller and the latent image carrier. However, such alternating electric field may not be sufficient to develop a higher resolution image composed of uniform tiny dots because the toner may not be stably supplied to a latent image.
Further, in the one-component developing method, toner receives greater stress from the toner regulating member when the thin layer of toner is formed on the developing roller, and thereby toner circulating in the development unit may degrade in a shorter time. If toner degrades, the thin layer of toner may not be formed uniformly on the developing roller. Accordingly, the one-component developing method may not be suitable for image forming apparatuses for high speed printing or extending durability of image forming apparatuses.
One related art reference Japanese Patent Application Publication H03 (1991)-100575 describes a kind of hybrid method which combines the two-component developing method and the one-component developing method to reduce drawbacks of the one-component developing method for some degree although the size and number of parts of the development unit increases. However, such a hybrid method may also have a drawback that may occur at a developing area as similar to the one-component developing method, by which a higher resolution image composed of uniform tiny dots may not be reliably formed.
Further, another related art reference Japanese Patent Application Publication H03 (1991)-113474 describes a method for developing a higher resolution image composed of uniform tiny dots, in which a wire supplied with a bias voltage having high frequency wave is disposed at a developing area so as to form a toner cloud over the developing area. Although such development unit may stably produce higher quality image, the development unit may have a complex configuration.
Further, another related art reference Japanese Patent Application Publication H03 (1991)-21967 describes a method for forming a toner cloud efficiently and stably, in which an electric field curtain is generated on a rotating roller. Such a development unit can produce higher quality image while reducing the size of the development unit. However, in order to obtain higher quality image using such development unit, development conditions such as for example the electric field curtain may be limited to a given condition. If the development conditions are deviated from such given condition, the development unit may unpreferably produce a lower quality image.
When an image is composed of plurality of colors, such as yellow, magenta, and cyan, a plurality of toner images (such as for example a first toner image, a second toner image, and a third toner image) are sequentially formed or developed on one single image carrier. In such a developing configuration, it is required that a toner image formed on the single image carrier at an earlier timing is not disturbed by another toner image to be formed on the single image carrier at a later timing.
The non-contact type developing method or toner cloud method can be used for a developing unit for developing a plurality of toner images by forming an alternating electric field between the image carrier and a developing roller. However, such alternating electric field may tear off a toner image formed on the image carrier at an earlier timing from the image carrier, and a torn-off first color toner may intrude in a development unit using a second color toner. Such phenomenon may disturb the toner image formed on the image carrier, and furthermore can mix different colors of toner in one development unit. The intermixing is not desirable for producing a high quality image.
A developing method using a toner cloud can be used for forming a higher quality image, but the development conditions to obtain higher quality image, such as for example the electric field curtain, may be limited to a given condition, and if the development conditions are not set to such certain level, the development unit may unpreferably produce a lower quality image, as above mentioned.
Further, another related art reference Japanese Patent Application Publication 2002-341656 describes a configuration in which a mechanical movement of a toner carrying device is eliminated for a developing process, and in which the toner is electrostatically transported on a transport base plate using an alternating electric field having three phases or more to transport toner using electrostatic force. However, if the toner cannot be electrostatically transported in such development unit due to some reasons, toner may accumulate on the transport base plate, by which the development unit may not function normally.
Further, another related art reference Japanese Patent Application Publication 2004-286837 describes a configuration that a development unit has a fixed base plate and a toner carrying device, in which the toner carrying device moves on the fixed base plate so as to reduce the aforementioned drawbacks that toner may accumulate on the transport base plate. However, such a development unit needs a complex configuration, which is not preferable. Similarly, other configurations having some variations for development units for transporting toner or carrier particles are disclosed in Japanese Patent Application Publications 2003-15419, H09 (1997)-269661, and 2003-84560.
In light of such drawbacks, other related art references Japanese Patent Application Publications 2007-13387, 2007-13388, 2007-13389, and US Patent Application Publications 2007/0086811, 2007/0160395, 2008/0089720, and 2008/0089723 describe a development unit using a developing roller (or a toner carrying device) having electrodes of two phases. In such a development unit, an electric field generated between electrodes can be changed in a given time cycle to hop toner particles on or over the developing roller. Such hopping toner particles are transported to a developing area between the image carrier and the developing roller with a rotation movement of the developing roller. Such a developing roller having fine-pitched electrodes of two phases is referred as a flare roller, hereinafter. The flare roller has a surface protection layer used for insulating the electrodes.